For a wire rod obtained by hot rolling a high-carbon steel in the vicinity of eutectoid composition, it is general to perform a descaling process for removing the scale on the surface of the wire rod and a surface treatment process for enhancing sucking of lubricant at the time of wire drawing process, after transportation. Thereafter, the wire drawing process including a patenting treatment is performed one or two times, whereby a high-strength wire having small wire diameter can be obtained. The thus obtained wire is used for a steel cord for a tire, a belt cord for a conveyor belt, a saw wire of a cutting device and the like. Such a high-carbon steel wire rod is required to have a high primary wire drawing property (direct drawability). The primary wire drawing property represents an index indicating the degree of easiness of the wire drawing process in a structure state after a wire rod is hot-rolled. By performing the wire drawing process to the high-carbon steel wire rod having excellent primary wire drawing property, it is possible to manufacture a wire having the small wire diameter without applying an intermediate thermal treatment process.
Patent Document 1 discloses a technique in which a high-carbon steel containing carbon in a range of 0.6 to 1.0% by mass is subjected to four stages of cooling from a finishing temperature. This technique enables the surface of the wire rod to have a pearlite of 95 area % or more. This pearlite has an average nodule diameter P of 30 μm or lower and an average lamella distance S of 100 nm or more, and satisfies the following Equation 1, where P is expressed in μm and S is expressed in nm.F=(350.3/S0.5)+(130.3/P0.5)−51.7>0  [Equation 1]
In the technique of Patent Document 1, the cooling speed is controlled at an extremely slow speed of 2° C./s or lower in the third stage cooling using air-blast cooling after an hot drawing, thereby adjusting the pearlite block to have the average nodule diameter P of 30 μm or lower and an average lamella distance S of 100 nm or more. This makes it possible to avoid the wire breakage during the high-speed wire-drawing process, and further to prevent the decrease in lifetime of a die. However, this method requires a special configuration for the air-blast cooling. Further, in Patent Document 1, there is no description as to whether the ductility can be maintained without deterioration, even if the reduction of wire-drawing process increases.
For the high-carbon steel wire as described above, its workability is largely affected by a scale (oxide coating) adhered on the surface of the steel wire. Therefore, various studies have been made also on the scale.
The high-carbon steel wire rod such as a wire rod for a steel cord is required to have high productivity, and thus, is produced by using a mechanical descaling process. The wire rod is manufactured by hot rolling, and hence, the scale is adhered on its surface. The scale is required to have the following properties (1) through (3), which favorably contribute to the manufacturing of the wire rod.    (1) Extremely thin so as to be able to avoid scale loss.    (2) Not detached before a mechanical descaling process at customer's site from the viewpoint of prevention of rusting.    (3) The remaining scale ratio is as low as possible after the mechanical descaling process so as not to deteriorate a primary wire drawing property.
In terms of the scale, the adhesion property and the mechanical descaling property are conflicting with each other. More specifically, when the thickness of the scale decreases, the adhesion property increases while the mechanical descaling property deteriorates. Therefore, in the case of forming the scale thin, it is difficult to achieve both high adhesiveness and a high mechanical descaling property.
As a related art, Patent Document 2 discloses a wire rod having the properties (1) and (3) described above. This technique achieves a scale having thin and favorable release properties, by setting a percentage of FeO in the scale at 80% or lower. However, it does not take the property (2) into consideration. Based on experience of the present inventors, it was impossible to achieve the scale that remains unreleased after the hot-rolling process is completed, and that is not released during transportation, even if the percentage is set as described above.
Further, as a technique related to the scale adhesion property, Patent Document 3 discloses a high-carbon steel wire rod containing Ni in a range of 0.05 to 0.15% by mass, and having a degree of surface roughness thereof at 1.5 μm or lower. By using this high-carbon steel wire rod, it is possible to achieve high adhesiveness of secondary scale, and high mechanical descaling property before the wire drawing process. However, in a case of this method, addition of Ni is essential, and the object of this technique cannot be achieved without Ni addition. Further, it is not possible to secure the sufficient adhesiveness, even if Ni is added as a prerequisite. Such a scale property largely affects the primary wire drawing property of the steel material, and hence, there is a demand for development of the high-carbon steel wire rod having both a good steel structure and a favorable scale property.